Using X-ray techniques, a team "unpeeled" the layers of the famous painting to see how the Italian master achieved his barely perceptible graduation of tones from light to dark.

The technique used by Da Vinci and some other Renaissance painters to achieve this subtlety is called "sfumato," and unraveling it allowed the scientists to determined the composition and the thickness of the paint layers.

Philippe Walter, a senior scientist at the Paris-based Laboratoire du Centre de Recherche et de Restauration des Musees de France, told CNN: "This will help us to understand how Da Vinci made his materials... the amount of oil that was mixed with pigments, the nature of the organic materials, it will help art historians."

Walter and his colleagues used X-ray fluorescence (XRF) spectrometry to determine the composition and thickness of each painted layer of the Mona Lisa in the Louvre Museum in Paris, where the painting is normally kept behind bulletproof glass. Art historians believe it was painted by Da Vinci in 1503.

They found that some layers were as thin as one or two micrometers and that these layers increased in thickness to 30 to 40 micrometers in darker parts of the painting. A micrometer is one thousandth of one millimeter.

They believe this characterizes a technique of painting that uses a glaze, or very thin layer, to build up shadows in the face.

The manner in which Da Vinci painted flesh, "his softened transitions," were pioneering work in Italy at the end of the 15th century, say the researchers, and were linked to his creativity and his research to obtain new paint formulations.

Walter said it is almost impossible to see any brushstrokes on the Mona Lisa.

The research, which is reported in the journal Angewandte Chemie, also looked at several other Da Vinci paintings and could eventually help to determine when and how he painted some of his masterpieces.

However, Walter, added: "There is still plenty of mystery surrounding the Mona Lisa. This does not tell us why he painted, about his motivation, just about the materials."